1. Field of the Invention
This invention relates to resistance welding and, more particularly, to a control method of resistance welding which discriminates the material of a weldment in an initial welding period and thereafter continues and completes welding by changing weld conditions so that they meet the requirements of the material under process.
2. Description of the Prior Art
In the present-day production line of handling car bodies by the use of a number of welding robots, for example, an increasing number of cases include a process to which various weldments of different materials, such as bare steel plate and galvanized sheet iron, are subjected in a complicated order in accordance with a specification about products. To provide a good quality of welding under such circumstances as above it is necessary to change weld conditions depending upon a change of weldment.
Hitherto, in order to judge whether or not the surface of a weldment on the multiproduct-mixed production line is plated, an optical detecting system was sometimes used which uses generally an infrared ray.
However, because an optical sensor of the foregoing conventional system is located in a narrow space close to an electrode tip, it has the disadvantages that spatters produced at the time of welding, water, oil and dust given from the external, etc. adhere easily to the surface of the sensor, the sensor tends to be damaged due to contact with weldments or clamping tools, and the same is comparatively high-priced. Thus, the known systems are not feasible.
In practice, to weld galvanized sheet iron and bare steel plate supplied in a random order, for example, the weld conditions are adjusted so as to match with one weldment, e.g. galvanized sheet iron, whose weld conditions are severer than that of the other, so that the other weldment obeys the adjusted weld conditions. As a result, the galvanized sheet iron needs a larger current than the bare steel plate, whereas the bare steel plate is welded under excessive weld conditions. After all, the present-day industry performs welding unavoidably under the weld conditions which yield no inferior welding, but a somewhat bad external appearance and a little dust.
The remaining problem is that as the welding is performed repeatedly under excessive weld conditions for a long time against the bare steel plate, wear of the electrode tip is promoted due to voluminous generation of dust, for example, and the interval of dressing of the electrode tip and tip exchange work is made shorter. From the manufactural point of view which seeks to provide a high operating ratio, these time-consuming incidental tasks raise serious problems.